Fuel rail inner diameter question

[Catmaigne]

I'm making a custom fuel rail for a boosted LT1 where the goal somewhere around 600-700whp in a supercharged app. I know the stock 3/8" fuel lines will get the job done with dual 255s in tank (although they will be approaching their limit). However, I'm not sure if the inner diameter of the fuel rail extrusion I chose is too small and will create big pressure waves inside from the injectors opening and closing. The rail I'm considering is from Ross with an ID of .5" for use with -6an fittings so it's a perfect match for the current lines. I also plan on running the fuel rails in parallel and a 1:1 FMU to account for boost. The next rail size up is the Ross 8 and I think it's way too big with an ID of .69" (nearly twice the CSA). Would I be able to get away with the .5" rail diameter without any sort of damper or am I pushing my luck? Should I go with the larger diameter Ross 8 even though it's way bigger than I need? Is there such thing as having too much fuel in the rails (ie would it be harder on the fuel pumps to pressurize a larger volume)?

Some specs... yes, the stock LT1 rail is big (commonly modded to parallel to support a ton of power)
Cross sectional area
Stock LT1 rail (<5/8 ID) - .307
Edelbrock rail (9/16 ID) - .249
6an (1/2 ID) - .196
8an (11/16 ID) - .374

Any input is appreciated.

[Sir Yun]

(ie would it be harder on the fuel pumps to pressurize a larger volume)

No. Fluids are incompressible so the pump will not ''notice'' a thing IMHO.

The only reasons I can think of ( not an expert) to limit the volume in a fuel rail are

1: packaging & cost
2: heat soak : if you have a large volume of fuel in the rail it might become hotter in a stationary/heat soak situation and then the fuel temps will be higher for a longer time when the flow picks up.

[NORSK]

The bigger the better,i have never used anything smaller than 8an fuel lines/rail on the 4 and 6cyl Turbo engines i have worked With

[Rick Finsta]

I run -6 Ross rails on my setup which is similarly sized (I'm less hp but on E85 so targeted 750hp gas to give myself a little headroom) with no pressure fluctuation issues in a batch fire system; gauge reads rock solid. I run a -8 from the pump (Fuelab Prodigy) to a -8/-6/-6 Y into the rear of the rails, then come out the front with -6 into the regulator (one side goes through a 3/8" ID straight-through flex fuel sensor first) and out the regulator -8 back to the tank.

I've seen a lot of guys running single inlet/outlet with a crossover line at similar power levels without issue but I think Y-splitting is a better practice.

[vwchuck]

Pressure fluctuations in the rail have nothing to do with how much HP you are making. The larger the ID the better to dampen pressure fluctuations. If you actually had the capability to measure what was going on in the rail you would see at different RPM you will have higher or lower instantaneous pressure at the injector injector inlet but it will change as RPM's change. So unless you have a problem at a steady state operating point I wouldn't worry about it as auto engines are transient machines.

[Rick Finsta]

I think you've contradicted yourself; too small of a volume in the rail feeding too high of HP (large injectors and high PW) will lead to fuel pressure fluctuations in the rail; rpm is obviously just affecting the total time the injectors are acting as leaks but all the factors are inter-connected. It is widely accepted batch fire systems are worse about this when plumbed in a bank-to-bank configuration where each rail fires all the injectors at once; I realize "widely accepted" does not always make for "objective truth" so if you can expand on your point I'm eager to learn.

Am I thinking about this like a gas not a liquid or something?

[clshore]

Seeing pressure in the rail vs RPM is not so hard, pressure sensors are cheap and easy to interface to low cost PC scopes.
The problem is finding a sensor with enough response bandwidth to faithfully reproduce what's happening.

Assuming a V-8 with 2 rails, in each rail batch fire events occur every other revolution, while sequential fire events occur twice per revolution.
So at 7200 RPM you get 120 revolutions/second, 60 pulses/sec for batch, 240 pulses/sec for sequential.

The sensor should have at least 1 KHz bandwidth to show what's going on.
A sensor on each end of the rail allows you to see if there are issues with standing and traveling waves and such inside the rails by using
a dual trace display.

[Rick Finsta]

Some systems allow for more injections per revolution as well but only within the confines of the injectors' flow characteristics, obviously. So the resolution needs could be an order of magnitude higher for some setups.

[clshore]

+1 on that, too bad the higher bandwidth sensors are so expensive.
I wonder if a damper could be fitted (like ones used to eliminate water hammer in plumbing systems).
Or maybe if a system had a resonant characteristic, you could plumb a resonant stub to eliminate it, like on some exhaust systems (Honda S2000)

Edit: Found this one:
Volvo V70 fuel damper:
http://www.carid.com/1998-volvo-v70-fue ... 24575.html

[MadBill]

<EDIT> Penned before preceding post. Here it is anyway...>
As Sir Yun points out, liquid fuel for all practical purposes is incompressible. This means that whether the rail hold six ounce or six gallons is irrelevant (except for the minute expansion of the far larger container). If you want to damp pressure pulses, you will need an accumulator in the system, like the air space at the top of a water tank to prevent water hammer.

A different issue arises if two or more lines are tapped in down the length of an undersized rail; pressure drop along such a line could mean that the far end sees a lower pressure at high demand.

[Rick Finsta]

In my system I saw pulsing on my gauge when I had air trapped in the line to the gauge (regulator is low in front of the intake manifold and gauge is in front of the windshield at the cowl). I bled the line and now I get rock solid readings. Perhaps the springiness of the air allows the gauge needle to move at a harmonic of the pulses in the rail? I raised an eyebrow at the behavior but didn't give it much thought. It might have been the air actually affected how the regulator was working as well I suppose.